1. Field of the Invention
The present invention relates to a semiconductor nonvolatile memory, and particularly to a decode circuit suitable for an electrically data reprogrammable flash memory.
2. Description of the Related Art
An EEPROM has been known as an electrically erasable programmable semiconductor nonvolatile memory. A general EEPROM takes a stacked structure in which a memory cell transistor has a floating gate electrode and a control gate electrode. Upon data erasure, a boost or step-up level (VPP: about 12V) higher than a power supply level (VCC) used in a normal circuit is applied to a control gate electrode (WL) to pull or draw out electrical charges from a flowing gate electrode, thereby controlling the amount of the electrical charges in the floating gate electrode. That is, the amount of the electrical charges in the floating gate electrode is reduced to thereby bring the memory cell transistor into conduction when the power supply level (VCC) is applied to the corresponding control gate electrode (WL).
Upon reading, the control gate electrode (WL) is set to the power supply level (VCC) and a decision as to whether data is 1 or 0 is made according to conduction and non-conduction of the memory cell transistor. Thus, two cases arise in which the power supply level (VCC) is applied to the control gate electrode (WL) according to an operation mode and the step-up level (VPP) is applied thereto according to an operation mode.
FIG. 1 is a block diagram showing a control gate electrode type decode circuit (decode circuit) of a batch erasable programmable EEPROM (Flash EEPROM). FIGS. 2 through 5 are respectively configurational diagrams of respective circuits used in the decode circuit.
The decode circuit 1 comprises a predecode circuit 18 which inputs address signals A<1:0> and a control signal /CHIP brought to a ground level (VSS) at batch erasure, a redundant element 10 which holds and outputs a redundancy replacement flag (RDDEN) and redundant relief addresses (RA) set to a power supply level (VCC) where redundancy replacement is required, a redundancy determination circuit 12 which inputs the outputs (RA<1:0> and /RA<1:0>) of the redundant element and the outputs (XA<1:0> and /XA<1:0>) of the predecode circuit 18, a redundancy selector 14 which inputs the outputs (RXA<1:0>) of the redundancy determination circuit 12, the outputs (XA<1:0> and /XA<1:0>) of the predecode circuit 18 and the control signal /CHIP, a decoder array 16 which inputs the outputs (XEN and RXEN) of the redundancy selector 14, the outputs (XA<1:0> and /XA<1:0>) of the predecode circuit 18 and a control signal (ERASE), and a charge pump circuit 20 which supplies a boost or step-up level (VPP) to a boost or step-up power supply line (VEP) when the control signal ERASE is of the power supply level (VCC), and supplies the power supply level (VCC) to the step-up power supply line (VEP) when the control signal ERASE is of the ground level (VSS).
The decoder array 16 comprises a plurality of decoders (XDEC) 50 through 56 each of which inputs one of the outputs XA<0> and /XA<0> of the predecode circuit 18 and one of the outputs XA<1> and /XA<1> thereof, and the output XEN of the corresponding redundancy selector 14, a redundant decoder (RXDEC) 58 which inputs the output RXEN of the redundancy determination circuit 12, and a level shifter (LS1) which inputs the control signal (ERASE).
Each of the decoders (XDEC and RXDEC) comprises a logic gate (NA) which decodes each address, an inverter (INV) which inputs the output of the logic gate (NA), a transfer gate (CM00) of which the source is connected to the output of the inverter (INV) and the drain is connected to its corresponding control gate electrode (WL), a level shifter (LS0) which inputs the output of the logic gate (NA) and the output of the inverter (INV), and a transfer gate (CM01) of which the source is connected to the output of the level shifter (LS0) and the drain is connected to its corresponding control gate electrode (WL).
The transfer gate (CM00) comprises a PMOS transistor whose gate is configured as the output (ER) of the level shifter LS1, and an NMOS transistor whose gate is configured as the output (ER) of the level shifter LS1.
The transfer gate (CM01) comprises a PMOS transistor whose gate is configured as the output (ER) of the level shifter LS1, and an NMOS transistor whose gate is configured as the output (ER) of the level shifter LS1.
The operation of the conventional decode circuit 1 will be explained below with being divided into a read operation (a), an erase operation (b) and a batch erase operation (c).
(a) Read Operation
When data is read from the EEPROM (Flash EEPROM), a control signal /CHIP is set to a power supply level (VCC) and a control signal ERASE is held at a ground level (VSS). In this condition, address signals A<1:0> are inputted. Owing to the setting of the control signal /CHIP to the power supply level (VCC) at this time, a step-up power supply line (VEP) assumes the power supply level (VCC) and the control signal ERASE is set to the ground level (VSS). Thus, the output ER of the level shifter LS1 results in the ground level (VSS) and the output /ER thereof assumes the power supply level (VCC).
The outputs (RDDEN, RA<1:0> and /RA<1:0>) of the redundant element 10 respectively hold a predetermined logic level. That is, when no redundancy replacement is required, the RDDEN is held at the ground level (VSS), whereas when the redundancy replacement is required, the RDDEN holds the power supply level (VCC) and redundancy relief address data holds a potential corresponding to a control gate electrode WL<m> (where m=0, 1, 2 and 3) that needs replacement.
When address signals A<1:0> are inputted, the predecode circuit 18 converts the address signal A<n> (where n=0 and 1) into complementary address signals XA<n> (where n=0 and 1) and /XA<n> (where n=0 and 1) and outputs them.
If the value of the output RA<n> (where n=0 and 1) of the redundant element 10 is of the power supply level (VCC), then the redundancy determination circuit 12 outputs the address signal XA<n> (where n=0 and 1) to the corresponding redundant address RXA<n> (where n=0 and 1), whereas if the value of the output RA<n> (where n=0 and 1) is of the ground level (VSS), then the redundancy determination circuit 12 outputs the address signal /XA<n> (where n=0 and 1) to the corresponding redundant address RXA<n> (where n=0 and 1).
That is, when the value of the output RXA<n> (where n=0 and 1) of the redundancy determination circuit 12 is of the power supply level (VCC), the value of the RA<n> (where n=0 and 1) assumes the power supply level (VCC) and the address signal XA<n> (where n=0 and 1) is brought to the power supply level (VCC). Alternatively, the value of the /RA<n> (where n=0 and 1) assumes the power supply level (VCC) and the address signal /XA<n> (where n=0 and 1) assumes the power supply level (VCC). Thus, the input address A<n> (where n=0 and 1) and the redundant relief address RA<n> (where n=0 and 1) are brought into coincidence.
When information (based on XA<1:0> and /XA<1:0> outputted from the predecoder 18 and RXA<1:0> outputted from the redundancy determination circuit 12) about the coincidence of the input addresses A<1:0> and the redundant relief addresses RA<1:0> is transmitted to the redundancy selector 14, the redundancy selector ANDs all the redundant addresses RXA<1:0> and the redundancy replacement flag RDDEN to thereby make a decision as to whether or not redundancy replacement is required. When the redundancy replacement is required, the redundancy selector outputs the power supply level (VCC) and the ground level (VSS) to the RXEN and XEN respectively. When no redundancy replacement is required, the redundancy selector outputs the ground level (VSS) and the power supply level (VCC) to the RXEN and XEN respectively.
Each of the decoders constituting the decoder arrays 50 through 58 ANDs one of the address signals (XA<0> and /XA<0>), one of the address signals XA<1> and /XA<1>, and the output XEN of the redundancy selector and thereby selects the corresponding control gate electrode WL<m> (where m=0, 1, 2 and 3). Further, the corresponding redundant control gate electrode RWL is selected according to the output RXEN of the redundancy selector.
When no redundancy replacement is required, for example, that is, the XEN is of the power supply level (VCC) and the RXEN is of the ground level (VSS), the input addresses A<1:0> are transmitted from the predecoder 18 to the decoders, and the result of a decision as to whether the redundancy replacement is required, is transmitted to each corresponding decoder via the predecoder circuit 18 and the redundancy determination circuit 12, whereby the control gate electrode WL<m> (where m=0, 1, 2 and 3) corresponding to the input addresses A<1:0> is selected.
When the redundancy replacement is required, that is, the XEN is of the ground level (VSS) and the RXEN is of the power supply level (VCC), the result of a decision as to whether the redundancy replacement is required, is transmitted to the corresponding decoder via the predecoder 18, redundancy determination circuit 12 and redundancy selector 14 so that the corresponding redundancy control gate electrode RWL is selected.
Since, at this time, the result of the decision as to whether the redundancy replacement is required, is transmitted to the corresponding decoder via the predecoder 18, redundancy determination circuit 12 and redundancy selector 14, the control gate electrodes WL<3:0> are brought to non-selection.
In the decoder which drives the selected control gate electrode, the output of a logic gate NA changes from the power supply level (VCC) to the ground level (VSS), and the output of an inverter INV changes from the ground level (VSS) to the power supply level (VCC). Since, at this time, a gate signal ER of a PMOS transistor constituting the transfer gate (CM00) and a gate signal /ER of an NMOS transistor constituting the transfer gate (CM00) are of the ground level (VSS) and the power supply level (VCC) respectively, the selected control gate electrode WL is driven to the power supply level (VCC) by the transistors constituting the transfer gate (CM00).
During the above operation, the control signal ERASE holds the ground level (VSS), and the charge pump circuit 20 inputted with the control signal ERASE supplies the power supply level (VCC) to the step-up power supply line (VEP).
(b) Erase Operation
When the data stored in the EEPROM (Flash EEPROM) is erased, a control signal /CHIP is set to a power supply level (VCC) and a control signal ERASE is held at a ground level (VSS). In this condition, address signals A<1:0> are inputted. Owing to the setting of the control signal /CHIP to the power supply level (VCC) at this time, a step-up power supply line (VEP) assumes the power supply level (VCC) and the control signal ERASE is set to the ground level (VSS). Thus, the output ER of the level shifter LS1 takes the ground level (VSS) and the output /ER thereof assumes the power supply level (VCC). The outputs (RDDEN, RA<1:0> and /RA<1:0>) of the redundant element 10 respectively hold a predetermined logic level.
When no redundancy replacement is required, the RDDEN holds the ground level (VSS)). When the redundancy replacement is required, the RDDEN holds the power supply level (VCC)), redundancy relief address data holds a potential corresponding to a control gate electrode WL<m> (where m=0, 1, 2 and 3) that needs replacement.
When the address signals A<1:0> are inputted, the predecode circuit 18 converts the address signal A<n> (where n=0 and 1) into complementary address signals XA<n> (where n=0 and 1) and /XA<n> (where n=0 and 1) and outputs them.
If the value of the output RA<n> (where n=0 and 1) of the redundant element 10 is of the power supply level (VCC), then the redundancy determination circuit 12 outputs the address signal XA<n> (where n=0 and 1) to the corresponding redundant address RXA<n> (where n=0 and 1), whereas if the value of the output RA<n> (where n=0 and 1) is of the ground level (VSS), then the redundancy determination circuit 12 outputs the address signal /XA<n> (where n=0 and 1) to the corresponding redundant address RXA<n> (where n=0 and 1).
When the value of the output RXA<n> (where n=0 and 1) of the redundancy determination circuit 12 is of the power supply level (VCC), the value of the RA<n> (where n=0 and 1) assumes the power supply level (VCC) and the address signal XA<n> (where n=0 and 1) is brought to the power supply level (VCC). Alternatively, the value of the /RA<n> (where n=0 and 1) assumes the power supply level (VCC) and the address signal /XA<n> (where n=0 and 1) assumes the power supply level (VCC). Thus, the input address A<n> (where n=0 and 1) and the redundant relief address RA<n> (where n=0 and 1) are brought into coincidence.
When information about the coincidence of the input addresses A<1:0> and the redundant relief addresses RA<1:0> is transmitted to the redundancy selector 14 via the predecoder 18 and the redundancy determination circuit 12, the redundancy selector 14 ANDs all the redundant addresses RXA<1:0> and the redundancy replacement flag RDDEN to thereby make a decision as to whether or not redundancy replacement is required. When the redundancy replacement is required, the redundancy selector outputs the power supply level (VCC) and the ground level (VSS) to the RXEN and XEN respectively. When no redundancy replacement is required, the redundancy selector outputs the ground level (VSS) and the power supply level (VCC) to the RXEN and XEN respectively.
Each of the decoders constituting the decoder array 16 ANDs one of the address signals XA<0> and /XA<0>, one of the address signals XA<1> and /XA<1>, and the output XEN of the redundancy selector 14 and thereby selects the corresponding control gate electrode WL<m> (where m=0, 1, 2 and 3). Further, the corresponding redundant control gate electrode RWL is selected according to the output RXEN of the redundancy selector 14.
When no redundancy replacement is required, for example, that is, the XEN is of the power supply level (VCC) and the RXEN is of the ground level (VSS), the input addresses A<1:0> are transmitted from the predecoder 18 to the decoders, and the result of a decision as to whether the redundancy replacement is required, is transmitted to the corresponding decoder via the predecoder 18, the redundancy determination circuit 12 and the redundancy selector 14, whereby the control gate electrode WL<m> (where m=0, 1, 2 and 3) corresponding to the input addresses A<1:0> is selected.
When the redundancy replacement is required, that is, the XEN is of the ground level (VSS) and the RXEN is of the power supply level (VCC), the result of a decision as to whether the redundancy replacement is required, is transmitted to the corresponding decoder via the predecoder 18, the redundancy determination circuit 12 and the redundancy selector 14 so that the corresponding redundancy control gate electrode RWL is selected.
Since, at this time, the result of the decision as to whether the redundancy replacement is required, is transmitted to the corresponding decoder via the predecoder 18, redundancy determination circuit 12 and redundancy selector 14, the control gate electrodes WL<3:0> are brought to non-selection.
When the control signal ERASE is next raised from the ground level (VSS) to the power supply level (VCC), the charge pump circuit 20 inputted with the control signal ERASE supplies a step-up level (VPP) to its corresponding step-up power supply line (VEP), and the output /ER of the level shifter (LS1) is changed to the ground level (VSS) and the output ER thereof is transitioned to the step-up level (VPP).
Owing to the supply of the step-up level (VPP) to the step-up power supply line (VEP), the output of the level shifter (LS0) in the decoder which drives the selected control gate electrode, is changed to the step-up level (VPP) and the output /ER thereof is brought to the ground level (VSS), whereby the selected control gate electrode is driven to the step-up level (VPP) through the corresponding level shifter (LS0) and transfer gate (CM01).
(c) Batch Erase Operation
When the data of the EEPROM (Flash EEPROM) is collectively erased, a control signal /CHIP is first set to a power supply level (VCC) and a control signal ERASE is held at a ground level (VSS). Owing to the setting of the control signal /CHIP to the power supply level (VCC) at this time, a step-up power supply line (VEP) assumes the power supply level (VCC) and the control signal ERASE is set to the ground level (VSS). Thus, the output ER of the level shifter LS1 takes the ground level (VSS) and the output /ER thereof assumes the power supply level (VCC). The outputs (RDDEN, RA<1:0> and /RA<1:0>) of the redundant element respectively hold a predetermined logic level.
When no redundancy replacement is required, the RDDEN holds the ground level (VSS). When the redundancy replacement is required, the RDDEN holds the power supply level (VCC), redundancy relief address data holds a potential corresponding to a control gate electrode WL<m> (where m=0, 1, 2 and 3) that needs replacement.
When the control signal /CHIP is transitioned to the ground level (VSS) in this condition, the predecode circuit 18 outputs the power supply level (VCC) to both of complementary address signals XA<n> (where n=0 and 1) and /XA<n> (where n=0 and 1).
Since both of the complementary address signals XA<n> (where n=0 and 1) and /XA<n> (where n=0 and 1) are of the power supply level (VCC), the redundancy determination circuit 12 outputs the power supply level (VCC) to the redundant addresses RXA<1:0> without depending on the output RA<n> (where n=0 and 1) of the redundant element 10. Since the control signal /CHIP is of the ground level (VSS), the redundancy selector 14 outputs the power supply level (VCC) to both of the XEN and RXEN.
On the other hand, each of the decoders constituting the decoder array 16 ANDs one of the address signals XA<0> and /XA<0>, one of the address signals XA<1> and /XA<1>, and the output XEN of the redundancy selector 14 and thereby selects the corresponding control gate electrode WL<m> (where m=0, 1, 2 and 3). Further, the corresponding redundant control gate electrode RWL is selected according to the output RXEN of the redundancy selector 14. However, all the control gate electrodes WL<3:0> and the redundant control gate electrode RWL are selected to bring all of the inputs of the decoders and redundant decoder to the power supply level (VCC).
In the decoder which drives the selected control gate electrode, the output of a logic gate NA changes from the power supply level (VCC) to the ground level (VSS), the output of an inverter INV changes from the ground level (VSS) to the power supply level (VCC), and the output of a level shifter (LS0) is transitioned to the power supply level (VCC).
When the control signal ERASE is next raised from the ground level (VSS) to the power supply level (VCC), the charge pump circuit 20 supplies a step-up level (VPP) to its corresponding step-up power supply line (VEP), and the output /ER of a level shifter (LS1) is changed to the ground level (VSS) and the output ER thereof is transitioned to the step-up level (VPP).
Owing to the supply of the step-up level (VPP) to the step-up power supply line (VEP), the output of the level shifter (LS0) in the decoder which drives the selected control gate electrode, is changed to the step-up level (VPP) and the output /ER thereof is brought to the ground level (VSS), whereby the selected control gate electrode is driven to the step-up level (VPP) through the corresponding level shifter (LS0) and transfer gate (CM01).
Since the control gate electrode is driven to a step-up level (VPP: about 12V) higher than a power supply level (VCC) used in a normal circuit upon data erasure in the EEPROM (Flash EEPROM), all the MOS transistors connected to the control gate electrodes and the boost or step-up power supply line (VEP) need a withstand voltage greater than the step-up level (VPP: about 12V).
In general, high withstanding of each MOS transistor is realized by thickening a gate oxide film and lengthening a gate length to thereby relax an electric field between respective terminals of the MOS transistors. However, a problem arises in that the MOS transistors are reduced in drive capacity.
In the conventional decode circuit, the level shifters (LS0 and LS1) and the transfer gates (CM00 and CM01) are respectively made up of high-withstand MOS transistors. However, the control gate electrode at reading is driven through the transfer gate (CM00). Thus, the reduction in the drive capacity of each of the MOS transistors constituting the transfer gate (CM00) incurs a delay in the operation of the control gate electrode. This delay is noticeable in particular upon the rise of a control gate electrode (WL) by a P type MOS transistor lower in channel mobility.
As a method of suppressing the delay in the rise of the control gate electrode, there has been known a method of expanding a gate width of each of P type MOS transistors constituting a transfer gate (CM00) to thereby ensure drive capacity of the transfer gate (CM00). Since, however, the transfer gate (CM00) needs a long gate width for the purpose of its high withstanding and is required for each control gate electrode, an increase in layout area cannot be avoided.
Even in the case of either a case in which the control gate electrode WL<m> (where m=0, 1, 2 and 3) is selected or a case in which the control gate electrode WL<m> (where m=0, 1, 2 and 3) is not selected, the conventional decode circuit needs the transfer of the result of a decision as to whether redundancy replacement is required, to the corresponding decoder via a predecoder, a redundancy determination circuit and a redundancy selector in addition to a path through which the input address A<n> is transmitted. Therefore, a delay in reading occurs in the path.
In the conventional decode circuit, all the control gate electrodes (WL<3:0> and RWL) are driven to the step-up level (VPP) upon batch erasure regardless of whether the redundant relief is required. There is, however, a possibility that when a defect with a leak has occurred in the control gate electrode WL<m> (where m=0, 1, 2 and 3), for example, the control gate electrode WL<m> (where m=0, 1, 2 and 3) having such a leak will be driven to a step-up level (VPP), thus causing a reduction in the step-up level (VPP) due to the leak from the control gate electrode WL<m> (where m=0, 1, 2 and 3).
On the other hand, since a control gate electrode WL<i> (where i≠m) having no defect is also driven to the step-up level (VPP), there is a possibility that a failure will occur even in the control gate electrode WL<i> (where i≠m) having no defect where the step-up level (VPP) is reduced.